Interesting information about this chapter and introduction;- All organisms consists of cells. An adult human being has approximately 1, 00,000 billion cells, all originate from a single cell, the fertilized egg cell or zygote. Cells arise from free existing cells as a result of cell division (cell reproduction) .
Cells reproduce to produce new cells, chromosomes produce new chromosomes and offspring’s produce new offspring. All cells arise from preexisting cells this shows that the continuation of life, including all aspects of reproduction, is based on the reproduction of cells. We commonly refer cellular reproduction as cell division and it is a part of the whole life of a cell i.e. cell cycle.
Question#1) What is cell cycle and what are its main phases?
Answer: CELL CYCLE;-
Cell cycle is the series of events from the time a cell is produced until it completes mitosis and produces new cells. The cell cycle is the series of events that take place in eukaryotic cell leading to its division (replication).
Phases of cell cycle; – cell cycle are divided into two phases.
1) Interphase or preparatory phase: During interphase cell accumulate nutrients, perform metabolic activities at a very high rate and grows in size.
2) Cell division phase or mitotic phase; – During this phase cell divides into two daughter cells.
1) Interphase; – During interphase cell accumulate nutrients, perform metabolic activities at a very high rate and grows in size. It is period between the end of prophase and the start of next prophase.
Sub-Phases of interphase ;- Interphase may be divided into three sub-phases.
- a) G1 sub-Phase (gap 1 phase);- It is the first and longest sub phase of the inter-phase. During this newly produce cell increase in size and major chemical changes occurs that prepare cell for DNA replication and synthesis of RNA, ribosomes and several enzymes. It completes in few hours to several days depending on type of cell.
- b) S sub-phase (synthesis phase) ;- It is most vital event of cell cycle. DNA replication starts and completes during this step and various enzymes for replication also produces. During this cell duplicates its chromosomes. As a result each chromosome consist of two sister chromatids.
- c) G2 sub-phase;- The period between completion of DNA synthesis and the beginning of active mitosis is called G2 phase. In the G2 phase, cell prepares proteins that are essential for mitosis, mainly for the production of spindle fibers.
After the G2 phase of interphase, cell enters the division phase i.e. M phase. It is characterized by mitosis, in which cell divides into the two daughter cells.
Cells that have temporarily or permanently stopped dividing are said to have entered a state of quiescence, called G0 phase.
G0 phase (G-Zero);- G0 phase: In multicellular eukaryotes, cells enter G0 phase from G1 and stop dividing. Some cells remain in G0 for indefinite period e.g. neurons. Some cells enter G0 phase semi permanently e.g. some cells of liver and kidney. Many cells do not enter G0 and continue to divide throughout an organism’s life, e.g. epithelial cells.
Mitosis;- A type of somatic cell division that results in two daughter cells each having same number and kind of chromosome as in the parent cell.
This process was first observed by a German biologist Walther Fleming in 1880. Mitosis occurs only in eukaryotic cells. In multicellular organisms, the somatic cells undergo mitosis. Prokaryotic cells undergo a process similar to mitosis called binary fission. They do not undergo proper mitosis.
Phases of mitosis;- Mitosis process completes in two steps. 1) Karyokinesis. 2) Cytokinesis.
1) Karyokinesis;- (karyo from karyon means nucleus and kinesis means division). Karyokinesis is further divided into four phases. A) Prophase. B) Metaphase. C) Anaphase. D) Telophse.
- A) Prophase;- (pro means before and phase means appearance stage); It is the initial stage during which chromatin material condenses to produce chromosomes which appears in the form of threads. Each chromosome consists of two chromatids. Both are connected by centromere. A specific complex protein structure to which spindle fibers attached is called kinetochore. Centrosome of the cell consists of two centrioles, each centriole duplicate to form centrosome. Each centrosome move to each end of cell and produce spindle fibers (long thread or fibers like structures made of tubulin proteins helps in movement of chromosomes during cell division). In plant cell nucleus migrate to center. Nuclear membrane disappears. In plant cells centrioles are absent so fibers are produced by aggregation of tubulin proteins.
Metaphase;- (meta means after and phase means appearance stage); At this phase chromosomes arranged at the center of cell and form metaphase plate or equatorial plate. Microtubules from two opposite kinetochore attach with each other.
Anaphase (ana means upper) ;- The centromere of each chromosome divides due to the pull exerted by spindle fibers, thus each daughter chromosome moves to the opposite pole. At the end of anaphase, cell has succeeded in separating identical copies of chromosomes into two groups at the opposite poles. During this phase cytokinesis process somewhat started.
Telophase:- (Telo means end); This is the last and final phase and is the reversal of prophase. Nuclear envelopes forms around each set of chromosomes. Spindle fibers disappear and chromosomes unfold back into chromatin. Nuclear division completed.
cytokinesis ;- (cyto means cytoplasm of cell and kinesis means division); The division of cytoplasm of the cell is called cytokinesis.
Cytokinesis in animal cell;- In animal cells, cytokinesis occurs by a process known as cleavage. A cleavage furrow develops where the metaphase plate used to be. The furrow deepens and eventually pinches the parent cell into two daughter cells.
Cytokinesis in plant cell;- Cytokinesis in plant cells occurs differently. Vesicles derived from the Golgi apparatus move to the middle of cell and fuse to form a membrane-bounded disc called cell plate or phragmoplast. The plate grows outward and more vesicles fuse with it. Finally, membranes of cell plate fuse with plasma membrane and its contents join the parental cell wall. The result is two daughter cells, each bounded by its own plasma membrane and cell wall.
Question#2) Describe the importance of mitosis?
Answer: Significance or importance of mitosis;- significance of mitosis can be judged by its involvement in the following vital activities taking place in living organisms.
Development and growth: The number of cells within an organism increase by mitosis. This is the basis of the development of a multicellular body from a single cell i.e. zygote and also the basis of the growth of multicellular body.
Cell replacement;- Cells are constantly cast off, dying and being replaced by new ones in the skin, blood and digestive tract. The new formed cells are exact copies of the cells being replaced and so retain normal function. Mitosis ensure proper replacement of lost cells and tissues.
Regeneration: Some organisms can regenerate parts of their bodies. The production of new cells is achieved by mitosis. For example; sea star regenerates its lost arm and lizard regenerate its tail through mitosis.
Asexual reproduction: Some organisms produce genetically similar offspring through asexual reproduction. Mitosis is a mean of asexual reproduction. For example; hydra reproduces asexually by budding. The cells at the surface of hydra undergo mitosis and form a mass called bud. Mitosis continues in the cells of bud and it grows into a new individual. The same division happens during asexual reproduction (vegetative propagation) in plants.
Question#3) How chromosome moves on spindle fibers?
Answer; Microtubules lengthen and shorten by the addition or removal of tubulin dimers. Kinetochore microtubules shorten in the region of the kinetochore, pulling the chromosomes apart. Polar microtubules push against each other and thus, push the two centrosomes apart. This, in turn, separates the chromosomes. As a result the chromosomes move towards poles of cell.
Question#4) what is the difference between plants and animal mitosis?
Answer: 1) In animals spindle fibers are produced by centrioles. Plants lack centrioles and spindle fibers are produced by aggregation of tubulin proteins.
2) Cytokinesis takes place by enfolding of cell membrane in animal cells while in plant cells it occurs by expanding of cell plate.
Question#5) Errors in Mitosis leads to cancer? Justify the statement.
Answer: Cancer: A disease caused by an uncontrolled division of abnormal cells is called cancer or tumor. And the affected cells are called cancerous cells.
Errors in the control of mitosis may cause cancer. All cells have genes that control the timing and number of mitosis.
Causes of errors and mitotic cancer: Sometimes mutations occur in such genes and cells continue to divide. It results in growths of abnormal cells called tumors. The mutation may be due to radiations, chemicals, etc.
Types of tumors: As long as these tumors remain in their original location, they are called benign tumors. But if they invade other tissues, they are called malignant (cancerous) tumors and their cells are called cancer cells. Such tumors can send cancer cells to other parts in body where new tumors may form. This phenomenon is called metastasis (spreading of disease).
Question#6) Describe the events of meiosis? Describe the significance of meiosis?
Meiosis: (The word meiosis comes from Greek word ‘meioun’, meaning “to make smaller,” since it results in a reduction in chromosome number). Meiosis is the process by which one diploid (2n) eukaryotic cell divides to generate four haploid (1n) daughter cells.
Haploid Cell: The cell that contains half the number of chromosomes or unpaired chromosomes is called haploid cell.
Diploid cell: The cell that contains half the number of chromosomes or unpaired chromosomes is called diploid cell.
Discovery: Discovered by a German biologist Oscar Hertwig in 1876.
Phases: Meiosis process can be divided into four main phases.
1) Interphase or preparatory phase: This phase is similar to interphase of mitosis. Please see interphase of mitosis.
In meiosis I, the homologous chromosomes in a diploid cell separate and so two haploid daughter cells are produced. It is the step in meiosis that generates genetic variations. Meiosis I occurs in two main steps i.e. karyokinesis and cytokinesis. The karyokinesis of Meiosis I is subdivided into prophase I, metaphase I, anaphase I, and telophase I.
1) Prophase- I: Prophase-1 is the longest phase and usually account for 90 % or more of the time spent in meiosis. Basic changes occur in prophase-1 are as follows;
- a) Synapsis: The pairing of homologous chromosomes(chromosomes that are same in size and shape are called homologous chromosomes) is called synapsis. During this process chromosomes becomes visible and synapsis occurs to form tetrads (a pair of chromosome contain four chromatids called tetrads or called bivalent because it contain two chromosomes)
- b) Crossing over: The exchange of segments by non sister chromatids of homologous chromosomes is called crossing over. Chiasmata is the point of attachment of non sister chromatids of homologous chromosomes. The crossing over take place at this point. In 1911 , the American geneticist Thomas Hunt Morgan observed the phenomenon of crossing over in fruit fly Drosophila melanogaster.
- c) Variation: due to crossing over variation in genetic materials occurs.
- d) other changes: nuclear membrane disappears, spindle moves into center and the tetrads connected to spindle fibers by kinetochores (each tetrad consist of two kinetochores).
2) Metaphase-1: chromosomes arranged at metaphase plate or equatorial plate still in homologous phase. Spindle fibers from one pole of the cell attach to one chromosomes of each pair while spindle fibers from the opposite pole attach to the other homologue.
3) Anaphase-1: Kinetochore spindle fibers shorten. It results in pulling apart the chromosomes of each pair. Since one chromosome is pulled toward one pole, two haploid sets are formed. Each chromosome still contains a pair of sister chromatids. Anaphase-1 is a key event in creating new genetic combinations in sexually reproducing species.
Chromosomes arrive at the poles. Each pole now has half the number of chromosomes but each chromosome still consists of two chromatids. Spindle network disappears, and nuclear envelope is formed around each haploid set. Chromosomes uncoil back into chromatin.
3) Interphase-2 or interkinesis: After meiosis I both haploid daughter cells enter a period of rest known as interkinesis or interphase II. The interphase II is different from the interphase of mitosis and meiosis I. There is no S-phase and so there is no duplication of chromosomes during this stage. Only cells increase in size.
It is the second part of meiosis and is similar to mitosis. It is subdivided into prophase II, metaphase II, anaphase II, and telophase II.
1) Prophase II: Prophase II takes much less time compared to prophase I. In this prophase, nucleoli and nuclear envelope disappear and chromatin condenses. Centrioles move to the polar regions and make spindle fibres.
Metaphase II: In metaphase II, chromosomes attach with kinetochore spindle fibers and align at the equator of cell to form metaphase plate or equatorial plate.
Anaphase II: In anaphase II centromeres are cleaved and sister chromatids are pulled apart. The sister chromatids are now called sister chromosomes, and they are pulled toward opposing poles.
Telophase II: Telophase II is marked with uncoiling of chromosomes into chromatin. Nuclear envelopes reform; cleavage or cell wall formation eventually produces a total of 4 daughter cells, each with a haploid set of chromosomes.
SIGNIFICANCE OF MEIOSIS
The significance of meiosis for reproduction and inheritance was described in 1890 by German biologist August Weismann. He pointed out that meiosis was necessary not only to maintain the number of chromosomes in the next generation but also to produce variations in next generation.
1) Maintenance of the chromosome number in next generation: Meiosis produces cells with single number of chromosomes (haploid or unpaired chromosomes) in sex cells. When these haploid cells combined, they produce diploid cells in next generations and thus maintains the chromosome number constant generation after generation.
2) Reproduction in protozoans and fungi: Protozoans fungi produce spores by this process.
3) Alternation of generation: diploid sporophyte generation of plants produce haploid spores by meiosis. Which grow into haploid gametophyte generation. Gametophyte generation produces haploid gametes through mitosis. The gametes combine to produce diploid zygote. Zygote undergoes repeated mitosis to become diploid sporophyte.
4) Variations: Meiosis leads to variations (difference of individuals in various characters from their parents and members of the same species is called variation) in sexually reproducing species. During crossing over exchange of segments among homologous chromosomes occur changes in individuals.
ERRORS IN MEIOSIS: The normal separation or moving apart of chromosomes towards opposite poles of the cell during cell division ( anaphase-1 and anaphase. Note that separation of chromosomes during mitosis anaphase is also called disjunction) is called disjunction.
Sometimes the separation of chromosomes is not normal and it is called non-disjunction.
ERRORS IN MEIOSIS
During anaphase I, chromosomes separate and go to opposite poles while during anaphase II sister chromosomes separate. It is called disjunction. Sometimes the separation is not normal and it is called non-disjunction. This results in the production of gametes which have either more or less than the normal number of chromosomes. If such abnormal gamete fuses with a normal gamete, it results abnormal chromosome number in next generation, for example 47 or 45 chromosomes in humans. This can lead to various abnormalities like down’s syndrome.
Question#7) Contrast mitosis and meiosis emphasizing the events that lead to different outcomes?
Answer: COMPARISON BETWEEN MITOSIS AND MEIOSIS:
1) Similarities between mitosis and meiosis
Both involve DNA replication, multiple phases, chromosome pulling mechanism, etc.
|1) Occurs in somatic cells.||Occurs in germ cells(sex cells).|
|2) Involve single cell division.||Control two consecutive cell division.|
|3) Product of mitosis is two cells.||Product of meiosis is four cells.|
|4) Daughter cells contain diploid number of chromosomes.||Daughter cells have single number of chromosomes.|
|5) No genetic variation is produced in daughter cells.||Genetic variation is produced in daughter cells.|
|6) No synapsis occurs.||Synapsis occurs.|
|7) No crossing over occurs.||Crossing over occurs.|
Question#8) Describe Apoptosis and Necrosis?
Answer: Apoptosis: Apoptosis is a type of programmed cell death that remove unwanted cells from the body.
Events in Apoptosis:
**During apoptosis, cell shrinks and becomes rounded due to the breakdown of cytoskeleton by enzymes.
**Its chromatin undergoes condensation and nuclear envelope breaks.
**In this way, nucleus spreads in the form of several discrete chromatin bodies.
**Cell membrane makes irregular buds known as blebs. Blebs break off from the cell and are now called apoptotic bodies, which are then phagocytized by other cells.
Importance or significance:
**Apoptosis can occur when a cell is damaged or undergoes stress conditions.
**Apoptosis removes the damaged cell, preventing it from getting further nutrients, or to prevent the spread of infections.
**Apoptosis also gives advantages during development.
Examples: 1)during the formation of fingers, the cells between them undergo apoptosis and the digits separate.
2) The disappearance of tadpole tail in frogs during metamorphosis.
Necrosis is the accidental death of cells and living tissues.
Necrosis is less sequential than apoptosis.
Causes: There are many causes of necrosis including injury, infection, cancer, lack of oxygen, toxins, etc. Necrosis may occur when a cell is given hypoxic (with less oxygen) environments.
During necrosis, there is a release of special enzymes from lysosomes. Lysosomal enzymes break cellular components and may also be released outside cell to break surrounding cells. Cells that die by necrosis may also release harmful chemicals that damage other cells.
Spider bites also cause necrosis in some areas. Necrosis may be due to lack of proper care to a wound site.
Quetion#9) A Nerve cell does not divide after its formation. In which phase of cell cycle it is?
Answer: If a nerve cell does not divide after its formation it will be in G0 phase.
Question#10) What type of cell division occurs when our wounds are healed?
Answer: Mitosis occurs when our wounds are healed.
Question#11) How is cytokinesis different in plant cells as compared to animal cell?
Difference between Cytokinesis in Plant and Animal cells
|1. Cleavage cytokinesis occurs only in lower plants. Higher plants cells show cell plate cytokinesis.||1. It occurs through cleavage.|
|2. The middle part of spindle persists during cytokinesis. It forms a complex called phragmoplast. The phragmoplast is a plant cell specific structure that forms during late cytokinesis. It serves as a scaffold for cell plate assembly and subsequent formation of a new cell wall separating the two daughter cells.||2. Spindle degenerates during cytokinesis.
|3) cell plate forms.||3)Cell plate absent.|
|4) It starts with the formation of cell plate during telophase.||. It starts as a constriction in the cell membrane during late anaphase or early telophase.|
|5) The cell plate grows from the centre towards the lateral walls. So it is described as centrifugal.||The furrow deepens from the periphery to wards the centre and meets at the centre, So it is described as centripetal.|
|6) Wall formation occurs in the region of cytokinesis.||Wall formation is absent.|
Question#12) Plants do not make their gametes by meiosis. How is that?
Answer: Diploid sporophyte generation of plants produce haploid spores by meiosis. Which grow into haploid gametophyte generation. Gametophyte generation produces haploid gametes through mitosis. The gametes combine to produce diploid zygote. Zygote undergoes repeated mitosis to become diploid sporophyte.
Question#13) The S-phase of interphase is important and a cell can never divide without it.justify?
Answer: S sub-phase (synthesis phase) ;- It is most vital event of cell cycle. DNA replication starts and completes during this step and various enzymes for replication also produces. During this cell duplicates its chromosomes. As a result each chromosome consist of two sister chromatids. As most of the important events occur during this phase, so a cell can never divide without it.
Question#14) Define the following terms?
Non-sister chromatids : Either of the two chromatids of any of the paired homologous chromosomes.
Sister chromatids: Chromatids of the same chromosomes are called sister chromatids.
Kinetochore: The place on either side of the centromere to which the spindle fibers are attached during cell division is called kinetochore.
chromatids: One of the two strand of a chromosome is called chromatid.
Benign: Type of tumor that does not invade surrounding tissues or spread to other parts of the body.
M phase: The phase in cell cycle during which division of the cell take place is called mitotic phase or M-phase.
Phragmoplast: The cytoplasmic structures that forms at the equator of the after the chromosomes have divided during the anaphase of cell division and that indicates cell division.
Malignant: Type of tumor that invade surrounding tissues or spread to other parts of the body is called malignant tumor.
Chiasmata: Chiasmata is the point of attachment of non sister chromatids of homologous chromosomes.
Metaphase plate: A plane in the center of the cell in dividing cell on which the chromosomes are arranged during metaphase stage of the cell division.
Spindle fibers: These are protein filaments produced by centrioles on which the chromosomes moves during cell division.
G 0 phase: G-0 Phase or resting phase is a period in the cell cycle in which cell exists in a quiscent. During this phase the cell never divides nor preparing to divide.
Interphase: the preparatory phase of cell cycle during which a cell increases in size and produces all the materials requires for cell division is called interphase.
Centromere: The center of chromosome through which two chromatids are attached is called centromere.
Budding: Budding is a type of asexual reproduction in which an outgrowth appears from the parents body, then detaches and forms a new individual. “or” budding is the appearance of an outgrowth due to increase cell division.
UNDERSTANDING THE CONCEPTS
- What is cell cycle and what are its main phases?
Answer: please see question number 1.
- The S-phase of interphase is important and a cell can never divide without it. Justify.
Answer: please see question number 12.
- How would you state the events of prophase of mitosis?
Answer: please see question number 1.
- Make a list of the events of mitosis.
Answer: please see question number 1.
- How is mitosis significant?
Answer: please see question number 2.
- Describe the events that occur during the phases of meiosis-I.
Answer: please see question number 6.
- Describe the significance of meiosis, keeping in mind the applications of this type of cell cycle.
Answer: please see question number 6.
- Contrast mitosis and meiosis, emphasizing the events that lead to different outcomes.
Answer: please see question number 7.
- Describe necrosis and apoptosis.
Answer: please see question number 8.
- A nerve cell does not divide after its formation in cell cycle. In which phase of cell cycle it is?
Answer: please see question number 9.
- How is cytokinesis different in plant cells as compared to animal cell?
Answer: please see question number 11.
- What type of cell division (Type of cell cycle) occurs when our wounds are healed?
Answer: please see question number 10.
- Plants do not make their gametes by meiosis. How is that?
Answer: please see question number 12.